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Geological evolution of the Canning Basin, Western Australia

Abstract

Underlying an area of 430 (lOO km2 onshore and 165000 km2 offshore in Western Australia the Canning Basin contains about 10
000 m of Palaeozoic, Mesozoic, and Cainozoic sedimentary rocks. The Fitzroy Graben is the major structure in the basin. Faults
separate it from the Leveque and Lennard Shelves to the north, and a faulted hingeline separates it from the Broome Arch to
the south. The basin deepens again to the south between the arch and the Anketell and Tabletop Shelves, on the margin of the
Pilbara Block. The deeper parts of this irregular synclinal zone form the Kidson Sub-basin and the Willara Suh basin. The
floor of the basin generally deepens offshore towards the edge of the present-day continental shelf, where major depocentres
are the Bedout and Rowley Sub-basins-northeasterly trending depressions that owe their origin to continental rifling and seafloor
spreading. The Canning Basin sequence has been divided into 11 informal basin-wide intervals, each of which consists of a
number of roughly contemporaneous rock units. Each interval contains sediments deposited during one or more transgressions
or regressions of the sea, and the boundaries between them are typically unconformities or disconformities. The earliest record
of sedimentation is of Lower Ordovician shallow-marine clastic and carbonate rocks (interval 1) in the basin onshore. Earth
movements and prolonged erosion preceded the deposition of marine sediments-including widespread evaporites-and continental
red beds during the Early and Middle Devonian (interval 2l. Major subsidence of the Fitzroy Graben in the later Devonian coincided
with the deposition of turbidites, followed by finer sediments, adjacent to carbonate platforms (interval 3) in an environment
of recurrent diastrophism which culminated in earth movements correlated with the Alice Springs Orogeny. Marine clastics-interbedded
with glacial debris during the Late Carboniferous to Early Permian and with fluvial deposits in the east during the Late Permian
(interval 4)-accumulated, with only a brief hiatus, until the Triassic, when deltaic and fluvial sediments prograded across
them (interval 5l. Further earth movements preceded the deposition of deltaic sediments during the Early and Middle Jurassic
and marine clastics for much of the Late Jurassic (interval 6l. Late Tithonian to early Neocomian marine sedimentation in
the offshore subbasins (interval 7) was interrupted by earth movements that may correlate with the separation of the Indian
and Australian continents. Upper Neocomian deltaic sediments were inundated in the Aptian by a basin-wide transgression of
the sea, in which sedimentation may have persisted in the offshore sub-basins until the early Cenomanian (interval 8l. The
later sedimentary and diastrophic history of the basin is largely confined to the offshore sub-basins, where major transgressions
of the sea are recorded in the Turonian to early Paleocene (interval 9), late Paleocene to late Eocene (interval 10), and
early Miocene to Recent (interval 11 ). Oil exploration in the Canning Basin must now be regarded as high risk: from eighty
petroleum exploration wells drilled up to 1976, the most encouraging show to date is a few litres of oil from Lower Carboniferous
sandstone. Offshore, temperatures in the Palaeozoic of the Fitzroy Graben appear to be too high for oil generation to be still
continuing, whereas the Mesozoic section has generally not yet reached temperatures adequate for the generation of oil. The
best potential onshore rests with the testing of Fairfield Group sediments, Devonian reef trends, and Permian plays in the
southeast of the Fitzroy Graben. The unknown and virtually untested offshore Triassic section may also have some potential.